To edit or open your message, place your mouse pointer over the message and press right mouse button in the following pop-up menu choose corresponding option ('Open' or 'Edit'):
The difference between 'Open' and 'Edit' items is, that you cannot edit messages, stored in 'Inbox' folder, using 'Open' item (you can just view your messages), instead 'Edit' option enables to edit your messages in any folder.
See also: